Not applicable.
The present invention relates generally to the field of display devices such as liquid crystal display (xe2x80x9cLCDxe2x80x9d) devices and the like. More specifically, the present invention relates to a method of and apparatus for controlling contrast for such LCDs, especially active-matrix LCDs, while receiving large dynamic range video data.
An xe2x80x9cimagexe2x80x9d is a pattern of physical light. An xe2x80x9cimage output devicexe2x80x9d is a device that can provide an output defining an image. A xe2x80x9cdisplayxe2x80x9d is an image output device that provides information to an observer in a visible form. A xe2x80x9cliquid crystal displayxe2x80x9d (xe2x80x9cLCDxe2x80x9d) is a display device that includes a liquid crystal cell with a light transmission characteristic that can be controlled in parts of the cell by an array of light control units to cause presentation of an image. A xe2x80x9cliquid crystal cellxe2x80x9d is an enclosure containing a liquid crystal material. An xe2x80x9cactive-matrix liquid-crystal displayxe2x80x9d (xe2x80x9cAMLCDxe2x80x9d) is an LCD in which each light control unit has a nonlinear switching element that causes presentation of an image segment by controlling a light transmission characteristic of an adjacent part of the liquid crystal cell. An LCD can have a plurality of electrically-separated display regions, each display region also being known as a display cell, or when the regions designate a small portion of the display, each display region is known as a xe2x80x9cpixel.xe2x80x9d Each pixel in a high density display matrix, such as for LCDs, requires its own active (switching element) driver (e.g., a thin film transistor). The light control units can, for example, be binary control units.
In recent years, due to the great needs of avionics displays, LCD devices are more popularly used in avionics displays than other solid image display elements because of the low power consumption of the LCD elements. Also, personal computers, portable game machines, and other devices requiring a visual interface often use LCDs to display data. Such LCDs can be matrix addressed, such as an active-matrix LCD, but the use of a thin film transistor with every pixel in an active-matrix LCD is required for high resolution. Recently, color LCDs have come into common usage also. The increased usage of the color LCDs is partially because of their availability and a color pixel density of 100 to the inch can be easily achieved.
LCDs are generally classified into two categories: passive-matrix LCDs and active-matrix LCDs. Active-matrix LCDs are more popular than passive-matrix LCDs because of their excellent image quality, high speed, high contrast ratio (i.e., ratio of maximum to minimum luminance values in the LCD), and superior color quality. Although the passive-matrix LCDs are advantageously used for high-density integration because of their simple structures and lower manufacturing costs, the passive-matrix LCD elements have crosstalk to a non-selected cell, and an increase in resolution, which is an object of the high-density integration, cannot be achieved. In contrast to this, in the active-matrix LCDs, crosstalk to a non-selected cell can be suppressed without posing any problem, and an image having a high resolution can be obtained, thereby considerably improving image quality. In this manner, a large number of active-matrix LCDs have been used in recent years. Also, passive-matrix and active-matrix LCDs operate with a back light, which is typically a fluorescent lamp.
Both the active-matrix and passive-matrix LCDs are a matrix of row and column electrodes with a pixel at the intersection of each row and column. The active-matrix LCD provides a transistor at the intersection of each row and column electrode to greatly improve the voltage control of each pixel. The LCD is driven by providing the video voltage to the pixels one row at a time. The LCD is refreshed at a frequency that minimizes the flicker of the LCD, typically greater than 30-Hz. In a typical LCD architecture, the row electrodes are used to select the row which is to be driven and the column electrode provides the drive voltage that is used to determine the gray shade or level of the pixel at the intersection of the selected row and column. In a passive-matrix LCD, the root-mean-square voltage across the pixel, as determined by the select line voltage and the gray level voltage, determine the gray level of the pixel. In an active-matrix LCD, the gray level voltage delivered by the transistor at the pixel determines the gray level.
Both categories of LCDs require light rays from a back light to generate the colors. The back light generates an image plane of light beneath the LCD, which in turn generates the color display. In both passive-matrix and active-matrix systems, the color is generated by an array of color filters.
However, in these LCDs, the following problems are posed. The image quality of active-matrix LCDs is substandard at some contrast settings and viewing angles. Also, image quality changes as the contrast is changed. From a usability standpoint, there exists a considerable amount of dissatisfaction with the contrast and image quality of active-matrix LCDs. Contrast control works on a CRT and users desire that type of interface because they comfortable with it, and the display image is appealing. In a CRT, when the contrast is adjusted up and down it looks good and it adjusts the contrast as one would expect. The contrast control in a CRT is very smooth and very continuous. The situation of the LCD contrast being difficult to adjust in comparison to the CRT is directly related to the fact that an LCD has a limited number of shades of gray, e.g., 64 shades of gray, whereas a CRT essentially has infinite shades of analog. Thus, a need exists to obtain better image quality and better control of the LCD""s contrast of the video input to make it more closely resemble or match the quality that is obtained with a CRT when its contrast is adjusted. There is a desire to achieve that parody with an LCD when its video contrast is adjusted. A discussion of manual contrast control of CRTs can be found in most text books, for example, Bernard Grob, Basic Television Principles and Servicing, pp. 267-268 (4th Ed. 1975).
LCDs having the above drawbacks are not satisfactorily used in image display devices which are popularly used in avionics and industrial applications, especially in military aircraft; image display devices free from the above drawbacks are desired. To date, some of the attempted solutions to the problem have included classical contrast gain function, digital contrast to input video, and contrast changes. The classical contrast gain function requires brightness as a video adjustment. On LCDs, the brightness of the video is controlled by adjusting the back light. The contrast change solution controls the contrast by selecting from the existing shades of gray as determined by the LCD driver system.
The viewability of an image on an LCD is generally determined by the brightness and contrast of the LCD and video signal corresponding to the image. The luminance of each LCD pixel corresponds to the amplitude of the video signal for the pixel. High amplitudes typically correspond to very bright pixels, while low amplitudes generally correspond to dark pixels. The range between the minimum and maximum amplitudes and the corresponding degrees of luminance may be subdivided into an almost infinite number of luminance levels, reflecting subtleties of shading and color represented by the video signal. The brightness and contrast adjustments of the LCD, on the other hand, are essentially static. Conventionally, brightness corresponds to a direct current signal added to the video signal so that the overall signal level increases. As a result, the overall display becomes brighter. For CRT displays, the DC component is added to the video signal. For LCDs, the backlight system responds to the brightness control.
Contrast, on the other hand, relates to the amplification of the video signal. Thus, as contrast increases, bright pixels become very bright, while relatively dark pixels become only slightly brighter. Generally, the contrast of an LCD is the degree of difference in tone between the lightest and darkest areas in an LCD; contrast is also the subjective assessment of the difference in appearance of two parts of a field of view seen simultaneously or successively. Contrast is a function of liquid crystal molecule alignment, drive voltage, and viewing angle. The user is able to manually adjust the viewability of the picture image through contrast control. The contrast control is a manual control associated with picture-display devices that adjusts the contrast ratio of the reproduced picture/image on the display. The contrast control is normally an amplitude control for the picture signal. The contrast ratio is the ratio of the maximum to the minimum luminance values in an LCD or a portion thereof; in other words, the contrast ratio is the range of brightness between highlights and shadows of the reproduced picture/image on an LCD.
Conventional video displays, such as CRT displays, also typically have a wide dynamic range (i.e., a number of different and distinguishable colors and shades) for displaying each pixel with the appropriate degree of brightness according to the video signal and the brightness and contrast criteria. Small increases in amplitude cause small increases in brightness, regardless of whether the increase is due to a change the video signal or the brightness or contrast control. Consequently, subtle differences in the video signals induce subtle differences in the picture rendered by the display.
In some applications, however, subtle differences are not apparent to the user. For example, in some radar-based imaging applications, the dynamic range or peak-to-peak variation of the video signal information is relatively small. A CRT display shows variations in the video signal as slightly different shades. Where the variations are very small, the differences between different shades in the image may be so slight as to be nearly imperceptible.
This problem is compounded for various modern displays which do not provide the broad dynamic range of CRT displays. Limitations in a display""s dynamic range can restrict, or even negate, the display of subtleties in the image. For example, while the dynamic range of various LCDs varies according to type and manufacturer, LCDs generally have a limited dynamic range, particularly in comparison to CRT displays. A typical LCD exhibits a dynamic range limited to, for example, 64 or even 16 shades of gray.
For displays with limited dynamic range, effectively displaying and viewing minor variations in the data or information content is difficult, if not impossible. With limited dynamic range, slight variations in the video signal are commonly lumped into the same shade. As a result, variations in the video signal may not affect the rendered image at all, potentially obscuring vital information. Thus, it would be advantageous to provide a system for utilizing the available dynamic range of a display to enhance the presentation of data.
In view of the foregoing, a need exists for a display architecture capable of controlling the display""s contrast over a large dynamic range of video data at high resolution display rates for transmission to the active-matrix LCDs.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can only be gained by taking the entire specification, claims, drawings, and abstract as a whole.
A method for controlling contrast of a liquid crystal display (xe2x80x9cLCDxe2x80x9d) device (either passive-matrix or active-matrix) in which a gray scale is used while receiving large dynamic range of video data to be displayed by the LCD device, the gray scale having a finite number of shades of gray, the LCD device being characterized by a transfer function, the LCD device having a contrast control device for input by a user, the LCD device communicating with a drive voltage generator that supplies drive voltages V to the LCD device, the method comprising the steps of: providing a plurality of look-up tables, the plurality of look-up tables representing a plurality of contrast settings of the LCD device; and selecting a single look-up table from the plurality of look-up tables in response to the contrast setting selected by the user through the contrast control device to affect the transfer function of the LCD device, the single look-up table containing all shades of gray available on the gray scale with each contrast setting. The values of the drive voltages so that all shades of gray are available with each contrast setting. The transfer function is nonlinear and is defined by transmission T as a function of drive voltages V, and wherein the transfer function comprises a plurality of dynamic sets of drive voltages V and is not fixed to a single distribution of gray scale. The contrast setting is a function of a plurality of signals representative of the video data to be displayed by the LCD device, which include digital signals, analog signals, and modulated signals (e.g., pulse-width, amplitude modulated, etc.).
In addition, an apparatus is provided according to the present invention which implements the method of the present invention and includes a memory device containing a plurality of look-up tables, the plurality of look-up tables representing a plurality of contrast settings of the LCD device; and means for accessing the memory device to read or search through the plurality of look-up tables and for selecting a single look-up table from the plurality of look-up tables in response to the contrast setting selected by the user through the contrast control device to affect the transfer function of the LCD device, the single look-up table containing all shades of gray available on the gray scale with each contrast setting. The means for accessing includes, but is not limited to, a processor, counter, programmable logic device, field programmable gate array, a switch that has a counter built into it, either rotary or rocker, etc.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. The particular values and configurations discussed in this non-limiting discussion can be varied and are cited merely to illustrate an embodiment of the present invention, and are not intended to limit the scope of the present invention.